The present invention relates to solid-state imaging devices for use for example in a digital still camera, digital video camera, etc.
In recent years, performance of MOS solid-state imaging devices with which peripheral circuits can be formed into an on-chip system has been conspicuously improved and is spreading. As an amplification transistor is provided in each individual pixel of a MOS solid-state imaging device, a threshold variance of the amplification transistor from one pixel to another and kTC noise (thermal noise) at the time of reset are to cause fixed pattern noise and random noise in image. To remove these noises, CDS (correlated double sampling) operation is performed to read out only a light signal that becomes image signal by obtaining a difference between a reset level after reset and an output level after transfer of electric charges of pixel.
A description will be given below with respect to problems in the case where image is taken with using a MOS solid-state imaging device in which CDS operation is effected. When a very bright light source is reflected within an image taking region of the solid-state imaging device, an intense light is to impinge as result also on an input section of the amplification transistor of corresponding pixels. For this reason, a reset level output at the input section of the amplification transistor is changed for example due to a leak of accumulated electric charges whereby its dynamic range is suppressed. As a result, a phenomenon occurs at those pixels on which the intense light is incident that an output of image signal is lowered on the contrary by CDS operation (hereinafter referred to as “black sun phenomenon”). When for example an image of the sun is taken, an unnatural image is attained as a center portion of the sun results in a black dot. This problem can be solved in still pictures by providing a mechanical shutter. When taking a moving picture, however, use of the mechanical shutter at the same time is low in practicality as a means for solving the problem, since it becomes a serious demerit in securing the exposure time and/or frame rate.
Methods for suppressing the black sun phenomenon have been proposed for example in Japanese Patent Application Laid-Open 2000-287131. The one disclosed in the above publication proposes a method for suppressing the black sun phenomenon where change in output when outputting a reset level is detected so that, if it is determined as an occurrence of the black sun phenomenon, a predetermined value is written as a reset level output.
When an intense light is incident, however, its effect in some cases may also appear in an image region other than those pixels on which such intense light is incident. This will be referred to hereinafter as “transverse stripe phenomenon”. A major factor in the occurrence of the transverse stripe phenomenon will now be described. FIG. 1 schematically shows construction of a prior-art MOS solid-state imaging device. In the MOS solid-state imaging device as shown in FIG. 1, an amplification circuit section 350 is provided between a pixel section 300 and a CDS circuit section 360 so as to amplify pixel signal before it is inputted to the CDS circuit section 360. It is thereby possible to achieve a higher S/N, since it can be made less likely to be affected by noise occurring at a signal output circuit section 370 after the CDS circuit section 360.
What are denoted by 301 to 303 each are a unit pixel cell within the pixel section 300, and these are two-dimensionally arranged. 304 is a constant current supply provided for each column, constituting a source follower amplifier in combination with a transistor for source follower located within the pixel cell 301 to 303. A common gate electric potential 307 is supplied to and a common power supply wiring 306 is connected to the constant current supplies 304. The signal of each pixel 301 to 303 is read out row by row onto an output signal line 308 and is outputted to the outside through the amplification circuit section 350, the CDS circuit section 360, and the signal output circuit section 370.
When an intense light is incident on the pixel 302 in this case, an amplification circuit within the amplification circuit section 350 provided for the column where such intense light is incident is saturated and is affected so that, for example, it departs from its operation range. In the case where the employed amplification circuit is for example a constant current type amplification circuit where circuit current is constant without depending on input amplitude of the amplification circuit, if the circuit is saturated so as to depart from its operation range, the constant current characteristic of the circuit current is deteriorated so that in some cases the current consumed at the amplification circuit section 350 as a whole is changed. Generally, the power supply and/or the GND line of the amplification circuits of the respective columns of the amplification circuit section 350 are connected in common. Therefore, when the constant current characteristic of one amplification circuit is deteriorated so as to change the current consumed at the amplification circuit section 350 as a whole, an electric potential of the power supply and/or GND line is sharply changed due to parasitic resistance of the power supply and/or GND line. Accordingly, when an intense light is incident so as to saturate the amplification circuit of one column within the amplification circuit section 350, the electric potential of the power supply and/or GND line is sharply changed and, as a result, the outputs of unsaturated amplification circuits of other columns are also sharply changed.
A description will be given below by way of FIG. 2 typically showing a taking of image of a window chart with respect to the manner of effect on image when such an sharp output change of the amplification circuit occurs. Referring to FIG. 2, 401 is dark or an output region where the amplification circuit is not saturated, corresponding to a pixel region of the pixel cells 301 in FIG. 1. A light in excess of that makes the amplification circuit saturated is incident on a region indicated by 402, which corresponds to the pixel indicated by the pixel cell 302 in FIG. 1. 403 is a region similar to the region 401 which is dark or where a light of the degree by which the amplification circuit is not saturated is incident, corresponding to a pixel region indicated by the pixel cells 303 in FIG. 1. Since outputs of the amplification circuits of the columns of the region 403 of the same row as the region 402 are sharply changed as a result of change in the electric potential of the power supply and/or GND line as affected by the region 402 where the amplification circuit is saturated, an image in the form of a stripe results along a transverse direction as shown in FIG. 2.